The present invention is generally directed to the recording and reproduction of signals on a magnetic medium, particularly the positioning of a record/reproduce transducer head adjacent to a track of information on a magnetic recording tape. More specifically, the invention is directed to a system for dynamically positioning plural transducing heads adjacent to tracks of video signal information in accordance with a predicted shape for the tracks.
Information signals, for example video signals, are typically recorded on a magnetic medium, such as magnetic tape, in discrete tracks of information. In one type of recording system that is in widespread use for recording video signals, the magnetic tape is disposed around the periphery of a scanning drum and longitudinally transported relative thereto. One or more magnetic transducing heads rotate about the circumference of the drum. The tape follows a helical path around the drum, so that the rotating head transcribes a path, or track, along the tape that is disposed at an angle relative to the longitudinal direction of the tape. As the tape is transported around the drum at a predetermined speed, successive adjacent tracks are formed on the tape at that angle. During playback, if the tape is transported around the scanning drum at the same speed, the rotating transducing head will successively read the tracks in the order in which they were recorded, under ideal conditions.
However, due to varied conditions such as stretching of the tape, differences in the normal speed between one machine and another, etc., the transducing head may not be precisely positioned over a recorded track. As the location of the head moves away from the center of the track, the quality of the reproduced signal begins to degrade.
Accordingly, in order to faithfully reproduce individual tracks of video information, it is necessary to deflect the transducing head in a direction that is substantially transverse to its path of movement around the scanning drum. In other words, the transducing head must be moved in a direction that is parallel to the axis of the drum to enable it to remain adjacent to a particular track of recorded information. The position of the head in this direction is sometimes referred to as its "elevation."
Various techniques have been developed to control the elevation of the head to maintain it substantially centered over the recorded track. One popular technique applies a continuously oscillating dither signal to a control voltage that determines head elevation, to provide feedback information that enables the transducing head to be precisely positioned Exemplary servo systems which utilize a dither signal for controlling the elevation of the transducing head are disclosed in U.S. Pat. Nos. 4,151,570, 4,163,993 and 4,485,414, among others.
Under proper tracking conditions, i.e., when the transducing head is precisely centered over the track of recorded information that is being reproduced, an RF information signal from the head, for example a video signal, is at a maximum amplitude. When the head is displaced to one side or the other of the track, the amplitude of the RF signal decreases. When the dither signal is imposed upon the head elevation position control signal, it causes the head to slightly oscillate to either side of the recorded track of information, resulting in a sinusoidal envelope in the RF waveform from the transducing head. If the average position of the head is centered over the track of information being reproduced, this RF envelope has a symmetrical shape. If, however, the average position of the head is displaced from the track, the envelope will be asymmetrical with respect to the dither signal. More particularly, the magnitude of the reproduced RF signal will be lower when the dither oscillations are at one extreme than when they are at the other extreme. This asymmetry in the envelope can be detected and used to correct the position of the head.
Transducing head elevation servo systems which employ a dither signal or similar type of signal for providing feedback information relating to the alignment between the head and a track of recorded information utilize two different types of information from the dither signal to control the elevational position of the heads. In one portion of the control loop of the servo system, real-time information that is obtained from a detected dither signal is used to effect instantaneous position control of the head. In another portion of the control loop of the servo system, the feedback information obtained from the detected dither signal is averaged over several successive scans of the recorded tracks. This information provides an indication of the average shape and relative position of a track, and can be used to effect dynamic or high rate error correction of the head position. The present invention is particularly concerned with this latter aspect of head elevation servo systems, i.e., the dynamic or high rate error correction based upon a predicted shape of the recorded track. Therefore, the description which appears hereinafter will be primarily focused upon this aspect of the head elevation servo system.
In the dynamic correction of the head position to account for the shape of the track, the magnitude of the dither correction signal is sampled at several locations along the length of each track being scanned. For example, 10-15 samples might be made along the length of each track. Each sample provides an indication of the elevational position of the head, relative to the track, at the location of that sample. By averaging the samples for the respective locations over several successive scans of the tracks, information regarding the average shape of the track is obtained. Thus, for example, if each track is generally "S" shaped, the head will be displaced to one side of the track during an early part of its scan of that track and displaced to the other side of the track during the latter part of the scan, since the head generally tends to follow a linear path. However, by sampling and storing the detected dither signal which indicates the displacement of the head, and subsequently applying these stored values to the head positioning control signal during subsequent scans, the head can be dynamically positioned in accordance with the predicted shape of the track.
Typically, the dynamic correction of the head position is carried out by sequentially connecting storage capacitors to the detected dither signal during successive, respective portions of the track being scanned. The connections of the capacitors to the dither correction signal can be controlled on a time basis, in a manner analogous to the operation of a demultiplexer. Thus, if N storage capacitors are provided, each capacitor could be connected to the dither correction signal for 1/N of the total time required to scan a single track of recorded information, resulting in N samples. These samples are then successively applied to the head position control signal during subsequent scans of recorded tracks.
In some types of magnetic tape recorders, two or more heads are in contact with the tape at any one time to record or reproduce plural tracks of information simultaneously. For example, in a video tape recording machine a field of video information can be divided over several successive tracks. In this plural head type of arrangement, the two transducing heads are mounted on a common deflectable arm and moved in unison during elevational positioning. The heads can be mounted close enough to one another to scan two physically adjacent tracks on the tape. To diminish the likelihood of cross-talk between the two adjacent tracks, the two heads can be offset at slight angles in opposite respective directions to the axis of the tape. This offsetting of the heads in opposite directions is typically referred to as "cross-azimuth". Since the two heads are mounted on a common movable arm for elevational positioning, the dither signal that is imposed upon the elevation control signal influences both heads. It is possible to employ the dither feedback signal from only one of the heads to effect elevational position correction. Since the two heads are always moved in unison, any asymmetry in the RF envelope from one head should be indicative of track misalignment of the other head as well.
In order to accommodate differences in the distance between heads from one machine to another, it would be more preferable to average the dither envelope in the RF signals from both of the heads, and use this averaged signal to control elevational position. When this averaging technique is employed, however, a problem arises in connection with the sampling of the detected dither signal for dynamic correction purposes if more than one pair of heads is used to reproduce the recorded tracks of information. For example, if two pairs of heads are employed each pair is alternately in and out of contact with the tape during recording and reproduction. Furthermore, the two heads in a pair which are in simultaneous contact with the tape are spaced in the direction of their movement around the drum. As a result, one head comes into contact with the tape slightly before the other head in the pair, and the signal from the trailing head in the direction of the scan is slightly delayed relative to the leading head. Because of this slight delay between the signals from the two heads in one pair and the alternating arrangement of head pairs, it is possible that the leading head in one pair will come into contact with the tape and begin to reproduce a track of recorded information before the trailing head of the other pair has come out of contact with the tape. If the dither signals from these two heads in different pairs are averaged together, the result does not provide any meaningful information because it pertains to two different portions of two different tracks. Consequently, the first sample of the detected dither signal must be disregarded.
It is possible to disregard the first sample by simply clamping it to a ground reference potential. However, if the following samples of the detected dither signal have a significant non-zero value, it can be appreciated that there will be a sharp transition between the value of the first, grounded sample and the following samples. This sharp transition can result in a spike in the head position correction signal, which could take some period of time to settle out of the control loop.
Accordingly, it is desirable to be able to reliably predict the value of a missing sample of the track curve measuring system. Since the missing sample represents the first sample in a sequence, it is most preferable to predict the value of this missing sample on the basis of the next few samples which can be reliably measured.